Cable television systems (CATV) were initially deployed so that remotely located communities were allowed to place a receiver on a hilltop and to use coaxial cable and amplifiers to distribute received signals down to the town that otherwise had poor signal reception. These early systems brought the signal down from the antennas to a “head end” and then distributed the signals out from this point. Since the purpose was to distribute television channels throughout a community, the systems were designed to be one-way and did not have the capability to take information back from subscribers to the head end.
Over time, it was realized that the basic system infrastructure could be made to operate two-way with the addition of some new components. Two-way CATV was used for many years to carry back some locally generated video programming to the head end where it could be up-converted to a carrier frequency compatible with the normal television channels.
Definitions for CATV systems today call the normal broadcast direction from the head end to the subscribers the “forward path” and the direction from the subscribers back to the head end the “return path.” A good review of much of today's existing return path technology is contained in the book entitled Return Systems for Hybrid Fiber Coax Cable TV Networks by Donald Raskin and Dean Stoneback, hereby incorporated by reference as background information.
One innovation, which has become pervasive throughout the CATV industry over the past decade, is the introduction of fiber optics technology. Optical links have been used to break up the original tree and branch architecture of most CATV systems and to replace that with an architecture labeled Hybrid Fiber/Coax (HFC). In this approach, optical fibers connect the head end of the system to neighborhood nodes, and then coaxial cable is used to connect the neighborhood nodes to homes, businesses and the like in a small geographical area.
FIG. 1 shows the architecture of a HFC cable television system. Television programming and data from external sources are sent to the customers over the “forward path.” Television signals and data are sent from a head end 10 to multiple hubs 12 over optical link 11. At each hub 12, data is sent to multiple nodes 14 over optical links 13. At each node 14, the optical signals are converted to electrical signals and sent to customers over a coaxial cable 15 in the frequency range of 55 to 850 MHz.
Data or television programming from the customer to external destinations, also known as return signals or return data, are sent over the “return path.” Form the customer to the node, return signals are sent over the coaxial cable 15 in the frequency range of 5 to 42 MHz. At the node 14, the return signals are converted to optical signals and sent to the hub 12. The hub combines signals from multiple nodes 14 and sends the combined signals to the head end 10.
FIG. 2 is a block diagram of a digital return path 100 of a prior art HFC cable television system that uses conventional return path optical fiber links. As shown, analog return signals, which include signals generated by cable modems and set top boxes, are present on the coaxial cable 102 returning from the customer. The coaxial cable 102 is terminated at a node 24 where the analog return signals are converted to a digital representation by an A/D converter 112. The digital signal is used to modulate a optical data transmitter 114 and the resulting optical signal is sent over an optical fiber 106 to an intermediate hub 12. At the intermediate hub 12, the optical signal is detected by an optical receiver 122, and the detected digital signal is used to drive a D/A converter 124 whose output is the recovered analog return signals. These recovered analog return signals are then combined in an analog fashion with analog return signals from other nodes.
The analog return signals present on the coaxial cable 102 are typically a collection of independent signals. In the United States, because the analog return signals are in the frequency range of 5 to 42 MHz, the sampling rate of the A/D converter is about 100 mHz, slightly more than twice the highest frequency in the band. A 10-bit A/D converter operating at a sampling rate of 100 MHz is typically used for digitizing the return signals. As a result, data will be output from the A/D converter 112 at a rate of about 1 Gbps. Therefore, the optical data transmitter 114 and the optical data receiver 122 must be capable of transmitting and receiving optical signals at a rate of 1 Gbps or higher. The high transmission data rate results in more expensive equipment, or a lower transmission distance, or both. The high transmission data rate also limits the number of analog return signals that can be aggregated for transmission on the same optical fiber.
Accordingly, there exists a need for a method of and system for transmitting data at a lower data rate on the return path of a Hybrid Fiber Coaxial CATV system.